He seems to be doing nothing, but _____ he is just waiting for his chance.

A．actually

B．gradually

C．frequently

D．certainly

压力容器操作人员要经常巡回检查液面计谨防泄漏，特别是在冬季要防止液面计（）；若发现超过检验期、玻璃板（管）有裂纹或破碎、阀件固死、经常出现假液位等情况，应立即停止使用。